1. Field of the Invention
The present invention relates to an elemental analysis method and semiconductor device manufacturing method, and especially relates to a novel elemental analysis method for measuring and analyzing tiny amounts of heavy metal impurities mixed deep into a semiconductor substrate or a conductivity determining impurity dose introduced in a semiconductor substrate in a semiconductor integrated circuit device manufacturing process, etc. and a semiconductor device manufacturing method using the elemental analysis method.
2. Description of the Related Art
Heavy metal contamination, which likely causes defective characteristics in semiconductor integrated circuit devices, should be prevented in a semiconductor integrated circuit device manufacturing process. Particularly, in a manufacturing process of CCD (charge coupled device) type or MOS (metal oxide semiconductor) type solid-state image pick-up devices, heavy metal impurities, such as iron (Fe), nickel (Ni), copper (Cu), and chromium (Cr), mixed into a silicon substrate during the process cause defective characteristics called white defects on a display screen and directly and adversely affect manufacturing yield of the devices. Therefore, strict contamination control should be performed on manufacturing apparatuses in each manufacturing step of solid-state image pick-up devices.
Particularly, a silicon substrate surface is highly possibly subject to direct exposure to a processing atmosphere during ion implantation or plasma etching. As is well known, the ion implantation and plasma etching on a silicon substrate is preformed in a chamber. The chamber and members provided in the chamber are made of metal materials such as aluminum. The metal materials contain tiny amounts of the aforementioned heavy metal elements. The implanted ions and plasma has energy and sputters the metal materials constituting the chamber and members provided in the chamber during ion implantation or plasma etching. Some of the sputtered metal materials reach the surface of the silicon substrate. The heavy metal particles generated in the chamber as a result of such sputtering also have a certain level of energy. Therefore, there is always a risk of heavy metals entering the silicon substrate during ion implantation or plasma etching.
For the purpose of controlling the heavy metal contamination or comprehending the state of heavy metal contamination on the silicon substrate in the manufacturing process as described above, various elemental analysis methods are used in the semiconductor device manufacturing process. Among such elemental analysis methods, atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-MS), total reflection X-ray fluorescence analysis (TXRF), and secondary ion mass spectrometry (SIMS) are well known.
In atomic absorption spectrometry (hereafter referred to as the AAS method), a sample is irradiated with light having a wavelength in accordance with an element to be analyzed (target element) (for example approximately 200 to 850 nm) to quantify the target element contained in the sample. More specifically, a sample is heated and atomized, and then irradiated with light so that the atoms of the target element make the transition from the initial low energy level (ground level) to another excited level (excited state) by the irradiation light. The amount of the absorbed light having a characteristic wavelength corresponding to a difference in energy between these levels is measured to quantify the target element in the sample. The sample introduced in the analysis apparatus is in a liquid form (sample solution). When an object to be analyzed is in a solid form, the object is dissolved in a solvent to prepare a sample solution for use. Therefore, even a small amount of sample can be measured.
In inductively coupled plasma mass spectrometry (hereafter referred to as the ICP-MS method), a sample is ionized by inductively coupled plasma under an atmospheric pressure and generated ions are separated in accordance with a mass-charge ratio (mass/charge number) in a mass separation unit. Then, the number of ions that have entered a detector is measured as electric signals to qualitatively and quantitatively analyze the elements contained in the sample. The sample introduced in the analysis apparatus is in a liquid form (sample solution) and, for example, HCl, HF, HNO3, NH4OH, or H2O2 can be used as solvent. This method characteristically allows for quick multi-elemental analysis; many elements can be quantified at a level of ng/L (ppt: parts per trillion), which is lower than a detection lower limit of other elemental analysis method. Isotope ratio measurement is also available.
In total reflection X-ray fluorescence analysis (hereafter referred to as the TXRF method), a fluorescent X-ray is entered into a sample at an angle at which the fluorescent X-ray totally reflects at the surface of the sample so as to qualitatively/quantitatively analyze the elements contained in the sample. Inner orbital electrons of the atoms contained in the sample are excited by the entered fluorescent X-ray and make the transition to outer orbits, whereby the outer orbital electrons make the transition to the inner orbits where the excited electrons are present. The X-ray emitted then has energy equal to a difference in energy between the outer and inner orbit energy levels. The difference in energy is specific to the element. Therefore, the element species is identified by analyzing the energy of the X-ray emitted from the sample using an energy dispersive X-ray detector and the element is quantified based on the signal intensity. In this analysis method, the fluorescent X-ray entering the sample is totally reflected at the surface of the sample, significantly reducing a background scattered X-ray emitted from the sample along with the element-specific X-ray. Consequently, the signal intensity-to-background X-ray intensity ratio (S/N ratio) is improved and the characteristic X-ray spectrum of tiny amounts of substance elements can be observed with accuracy. However, in this analysis method, the fluorescent X-ray enters the sample surface at a total reflection angle of approximately 0.5 degree or smaller; therefore, the X-ray does not penetrate deep into the sample. Consequently, this method is suitable for obtaining the element information at a depth of approximately 1 to 100 nm from the sample surface.
Currently, the sample is pretreated by a vapor phase decomposition (VPD) technique to concentrate existing heavy metals for improved analysis sensitivity in the AAS, ICP-MS, and TXRF methods. Here, the VPD technique will be explained. For example, when a sample is a wafer having a silicon oxide film, first, only the oxide film on the wafer is decomposed with HF (hydrofluoric acid) vapor so that the wafer surface becomes hydrophobic. Then, the wafer surface is scanned with a liquid droplet so that the droplet absorbs the substances to be analyzed, such as heavy metals, remaining on the wafer surface and the droplet is collected. The collected droplet is dropped on a wafer and dried under reduced pressure to form a dried taint in which the substances to be analyzed are concentrated. The dried taint is measured for example by the total reflection fluorescent X-ray analysis. The VPD technique concentrates the elements to be analyzed and therefore particularly useful for detecting heavy metal and other elements that are extensively distributed on the wafer in a shallow, two-dimensional manner. Conversely, when heavy metal and other elements are present deep within the wafer (several tens μm), the VPD technique cannot be used to concentrate them. When heavy metal and other elements are scattered about the wafer surface, the dried taint of the droplet containing the elements to be analyzed may be scattered about more extensively than their original positions. In such a case, the concentration of element per unit area may be decreased and the sensitivity may accordingly be reduced. In addition, the sample is destroyed when the wafer surface is made hydrophobic.
On the other hand, in the semiconductor technical field, heavy metal elements can qualitatively and quantitatively be analyzed also by the secondary ion mass spectrometry (SIMS) analysis that is used for measuring an injection amount of ion-implanted conductivity determining impurities or by the time of flight SIMS (TOF-SIMS) analysis used for observing an uppermost surface. In SIMS analysis, the sample surface is irradiated with an ion beam such as O2+ and Cs+ having energy of several KeV to sputter the sample surface part and release the atoms and secondary ions at the sample surface in a vacuum. The released secondary ions are drawn by an electric field and subject to mass analysis using a magnetic field or high frequency electric field. Therefore, the in-depth distribution of the impurity element concentration can be known with accuracy. In this analysis method, a diameter of the ion beam entered into the sample surface or the analysis area resolution is as small as several tens nm to 10 μm. Therefore, it is a particularly useful analysis technique where the positions of heavy metals are known or where the heavy metals are distributed on the sample surface two-dimensionally in a sheet-like form.
Recently in the semiconductor technical field, it has been required to measure and control an actual injection amount of conductivity determining impurities ion-implanted in a semiconductor substrate with the measurement uncertainty of within several % as the semiconductor integrated circuit patterns become finer and the elements have a higher density. Known means for measuring the actual injection amount of conductivity determining impurities include a sheet resistance measurement, thermal wave method, SIMS analysis method, etc.
As is well known, the sheet resistance measurement is used to measure the resistance of a thin layer having a uniform thickness. A bulk resistance is expressed by resistivity×sample length/sample cross-sectional area while a sheet resistance is expressed by resistivity/sample thickness (unit: Ω/□). Therefore, the sheet resistance is not a simple indicator of the concentration of conductivity determining impurities contained in the layer but an indicator of the concentration of conductivity determining impurities electrically activated in the layer provided that the thickness of the layer is known.
On the other hand, the thermal wave method is used to estimate the injection amount using a property in which a magnitude of damage (lattice defect) introduced in a silicon substrate by ion implantation is proportional to the injection amount. When a silicon substrate receives a thermal shock by a laser beam, a degree of displacement (deformation) of the substrate surface varies depending on the presence/absence of damage. In this technique, the magnitude of damage introduced in the silicon substrate by ion implantation is estimated based on the change in the degree of displacement and the ion injection amount is estimated. However, the degree of displacement of the substrate surface is also affected by surface roughness that is increased by oxygen plasma ashing of a resist film or cleaning in the semiconductor device manufacturing process. Therefore, for estimating the injection amount in the substrate in a process that is subject to some substrate treatment other than ion implantation, such as plasma ashing and cleaning, in general, a reference substrate that is not subject to any treatment but ion implantation as described above is separately prepared and the magnitude of damage is estimated in relative comparison with the reference substrate. SIMS analysis is as described above. Japanese Laid-Open Patent Application Publication No. 2001-235436 discloses a conventional elemental analysis technique relating to the present application.